A known multiway valve comprises a main valve chamber in which a main valve body, controllable by a pressure medium, is axially displaceable and into which open at least one pressure line and one operating bore which may be isolated with the aid of the main valve body; a control valve chamber which is an axial extension of the main valve chamber and is separated from the latter by an axially adjustable diaphragm, the latter, on one side being firmly clamped to the valve casing, and being connected, on the other side, with the main valve body; a movable control valve body which, in one of its end positions provides a connection between a line connected with the pressure supply line and the control chamber and, in its other end position, provides a connection between the control valve chamber and a venting duct for the control system; the diaphragm and possibly also a respective end of the main valve body, comprising an area to which pressure may be admitted from the control valve chamber, which is larger than the area at the other end of the valve body where pressure may be admitted in the opposite direction.
Since the radial vent bore of the multi-way valve is located opposite to the connecting line and is connected with the diaphragm surface which is remote from the control valve chamber, it is possible that breakdown occurs if, at the moment of reversal, the full pressure P is also admitted to the venting passage. This temporary state may, for example, occur during the opening of the main valve chamber. Since at this moment also the auxiliary valve chamber is under the pressure P it follows that the diaphragm is loaded from both sides with the same pressure which in this case acts against approximately equal areas on the two diaphragm surfaces. If this is the case, an unstable state is created, that is to say, the diaphragm is locked at the beginning of, or during the process of reversal or--in the least favourable case--even anywhere between the end positions, preventing the valve from moving into its other end position. However, even in the absence of the least favourable condition, the locking of a diaphragm always entails considerable delays of switching. These are undesirable not only because of the delays as such which cause the time of valve reversal or the closing of the main valve body to be unduly long, but also because the reversal or closing speed which ought to be constant, varies depending on purely accidental conditions.
According to the present invention there is provided a multiway valve comprising a casing defining a main valve chamber, in which a main valve body, arranged to be controlled by a pressure medium, is axially displaceable and into which open at least one pressure bore and one operating bore the bores being selectively isolated from the main valve chamber by means of the main valve body, a control valve chamber arranged as an axial extension of the main valve chamber and isolated from the latter by means of an axially movable diaphragm which is clamped between the valve casing and the main valve body, a controllable control valve body which, in one end position, connects a connecting line, the latter communicating with the pressure bore, with the control valve chamber, and in its other end position provides a connection between a control vent bore and the control valve chamber, the area of the diaphragm, and where applicable of the respective end of the main valve body to which pressure is admitted being greater than the area to which pressure may be admitted at the other end of the main valve body, and means being provided to pre-tension the diaphragm in the closing direction of the main valve body.
Preferably the pre-tensioning means comprises a spring which is accommodated in the control valve chamber.
This pre-tension prevents the diaphragm or the main valve body respectively from being locked at accidental positions when equal pressures are admitted to either side of the diaphragm, because the spring is capable in any position of forcing the main valve body to assume its closing position. Moreover, the spring aids the movements of closing even under normal operating conditions, which means that the time of reversal may be maintained constant.
In a preferred embodiment of the invention the spring element is made of spring steel and has the form of a disc with a rosette shaped cut-out. The edge of this disc shaped spring is firmly clamped so that the inward extending arms provide the main valve body with the required pre-tension.
In another embodiment of the present invention an annular member, preferably consisting of a plastics material is provided between the spring element and the side of the diaphragm which projects into the control valve chamber. Since this annular member occupies a major part of the control valve chamber, the free space which is used up by the movement of closing is reduced, and the speed of reversal is correspondingly increased. The relative movements of the resilient arms of the spring element are absorbed by the annular member which, in consequence of its shape, not only ensures that it occupies a central position but also prevents the diaphragm from bulging upwards and possibly snapping out of the clamping elements.